As one of the most expressive features of the human face, eyes play an important role in interpreting and understanding a person's desires, needs, cognitive processes, and emotional states. The importance of eye gaze or simply gaze is implicitly acknowledged as it is used for inspection of our surroundings and is important in social interaction.
Reflections of light can be detected when observing a person's eye. Especially when the reflections are relatively distinct and confined in their expanse, they are also denoted glints; however, the term glints may also be used for any type of reflection or portion thereof.
Gaze estimation and tracking, also denoted eye tracking, are important for many applications including human attention analysis, human cognitive state analysis, gaze-based interactive user interfaces and gaze contingent graphical display. That is, for analysis and/or for interaction with machines such as dedicated apparatuses, general purpose computers with software, etc. Gaze estimation and tracking are used not only as means for persons with a disability preventing them from using conventional user interface means such as keyboards or pointing devices, but also for using gaze and eye movements in user interaction with machines.
Robust nonintrusive eye detection and tracking are therefore crucial to human computer interaction, attentive user interfaces and the understanding of human affective states. As the eye scans the environment or fixates on particular objects in the scene, an eye gaze tracker simultaneously localizes the eye position and tracks its movement over time to determine the direction of gaze. The term ‘scene’ is used for the physical field within which the gaze can be estimated or tracked by a device. The physical space can be any space in three dimensions or two dimensions, where the latter is typical for gaze directed to a display screen.
One class of methods for tracking eye gaze is based on corneal reflections of light. Corneal reflection gaze tracking systems project some pattern of light onto the cornea and then detect the position of the pupil or iris or other distinctive feature representing the eye's orientation relative to reflections of that light pattern. The reflections (also known as glints) of that light pattern are used as a spatial reference. Thus, a gaze tracking system determines e.g. the center of the pupil and the glints and the change in the distance and direction between the two as the eye is rotated. The orientation of the eyeball can be inferred from the differential motion of the pupil center relative to the glints. Visible light can be used, but near-infrared light is often employed, as users cannot see infrared light and are therefore not distracted by it.
The main components of a typical corneal reflection gaze tracking system comprise a video camera sensitive to near-infrared light, a near-infrared light source (often light-emitting diodes arranged in square configuration) mounted to emit light towards a person's eyes, and a computer system or other hardware system for analyzing images captured by the camera. Analyzing the images comprises detecting the position of the pupil or iris or other distinctive feature representing the eye's orientation and detecting reflections from the light pattern to determine the position of the pupil or iris or other distinctive feature relative to reflections of the light pattern which is used as a spatial reference. Image processing techniques comprising e.g. intensity threshold and edge detection are employed to identify the glints and the pupil from the image captured by the camera.
With so-called homography normalisation or similar methods it is possible to make an eye tracking apparatus that estimates where a person is looking in a way that is substantially invariant to the person's head movements. This means that it is possible to obtain a highly flexible eye tracker that is simple in its configuration and can be made compact and mobile.
Generally, gaze trackers based on corneal reflection are quite accurate and robust when used indoors. Most eye detection and gaze estimation methods used for gaze interaction rely on reflections (glints) of IR light sources on the cornea. Even though light conditions can to some degree be controlled by IR illuminators, the system still faces problems when used in less constrained settings (such as outdoors or in cars or when light comes from windows) since several indistinguishable reflections may appear on the cornea. In this situation it is not obvious which reflections belong to a predetermined configuration of light. The predetermined configuration of light is also denoted ‘system light’, whereas light from any other sources is denoted ‘spurious light’.